Optical frequency waveguide and transmission system

ABSTRACT

An optical communication system is disclosed in which electromagnetic energy is communicated from a source to a receiver through a gastight structure filled with gas, such as carbon dioxide, at a selected pressure. The energy is of a first frequency and with a critical power value Pc so that the energy which passes through the gas in trapped in the form of a constant diameter beam. The energy also includes modulated electromagnetic energy at a critical power value less that Pc, but at a frequency higher than the first frequency, whereby the modulated energy is also trapped in the beam as it passes through the gas-filled structure.

FIP81 J6 [72] inventors Appl. No.

Filed Patented Assignee Uuuctfbeav T 50 y OPTICAL FREQUENCY WAVEGUIDEAND TRANSMISSION SYSTEM 9 Claims, 7 Drawing Figs.

lnt. CL....

ao4b 9/00 FeldofSearch 250/227; 33 ll94.5; 350/96, 96 (W6); 250/199 [56]References Cited UNITED STATES PATENTS 3,3 86,043 5/1968 Marcatili et al3S0/96WG 3.386.787 6/1968 Kaplan 350/96WG 3,399,012 8/ l 968 Peters350/96WG Pn'mary Examiner-Robert L. Richardson Assistant Examiner-AlbertJ. Mayer Attorneys-T. H. Warden, Frederick J. Lees and John R.

Manning ABSTRACT: An optical communication system is disclosed in whichelectromagnetic energy is communicated from a source to a receiverthrough a gastight structure filled with gas. such as carbon dioxide, ata selected pressure. The energy is of a first frequency and with acritical power value I, so that the energy which passes through the gasin trapped in the form of a constant diameter beam. The energy alsoincludes modu lated electromagnetic energy at a critical power valueless that P but at a frequency higher than the first frequency, wherebythe modulated energy is also trapped in the beam as it passes throughthe gas-filled structure.

OPTICAL FREQUENCY WAVEGUIDE AND TRANSMISSION SYSTEM The described hereinrelates to a communication system and is a division of copending patentapplication entitled Optical Frequency Waveguide and TransmissionSystem," Ser. No. 494,739.

This invention relates to waveguides, and more particularly to anoptical frequency waveguide that is self-generating.

A beam of electromagnetic radiation when launched and directed throughany material necesarily diffracts. Beam spreading, which results fromdiffraction, between source and receiver limits achievable power densityappearing at the receiver and prevents very small beam transmission evenover short distances. The subject therefor of the present invention isdiffractionless transmission of electromagnetic radiation by forming adielectric waveguide between source and receiver. Diffractionlesstransmission finds application in power transmision, communication,bloodless surgery, machinery, and many other fields of endeavor.

Transmitting large quantities of electric power requires rathersubstantial transmission systems. Low frequency transmision necessarilyinvolves operating at elevated voltages which produce losses due tocorona. Direct current transmission of power also involves losses due toresistance heating. Power transmission can be accomplished at opticalfrequencies with minimal losses and is the subject of the presentinvention.

Optical frequency energy cannot be transmitted through the atmospherefor two obvious reasons. First, the possibility exists that someone orsomething such as an aircraft could intercept the beam. Such aninterceptor would be seriously injured or even destroyed. Second.atmospheric aberrations, due to changing weather conditions and thelike, could cause the beam of optical frequency energy to be diffractedsuch that the energy would be directed to some unwanted target causingdamage and interrupting power trammissions. In either event, it would beunsafe to transmit optical energy through the atmosphere. An immediatesuggestion would then be to confine the beam of energy within a pipe orother safe medium, such as a fibre optical or light pipe.

Light pipes could not be used for the transmission of large quantitiesof energy because the material from which the pipe is made is lossy andwould seriously attenuate the beam and might even melt. Furthermore, itwould be nearly impossible to make a flawless (flaws cause reflection orabsorb energy, which in turn produces local heating that may melt the"pipe) light pipe of any appreciable length, such as twenty feet, letalone several miles. If such a pipe could be fabricated, it would soondevelop fatal defects of crazing or cracking and such material does notlend itself to splicing in the field. At 'the junction of two sectionsreflections would result. The present invention, on the other hand,provides a safe medium to transport large quantities of opticalfrequency power with negligi-' ble loss. Furthermore, with thetransmission vehicle being a gas, such a system lends itself to easyrepair and in many instances is self-repairing, (i.e. if the gasoverheats or ionizes within the beam, it will be quickly replaced by theremaininggas in the system).

For the same reasons enumerated above, communication links utilizingoptical frequencies must also be conducted over a medium which thepresent invention provides. Additionally, the present invention permitsseveral such communication links to be conducted over a singlestructure; for the matter, a power link and several communication linksmay be conducted over a single structure.

Drawing a comparison between the present invention and comparablemicrowave waveguides, it is apparent that substantial savings can berealized with the present invention, for the elaborate machining,polishing, and even plating necessary in microwave components isobviated.

ln applications where lasers have been employed, such as surgery andfine machining, to cut with a powerfulbeam, the

tion. With the present invention, the laser can be placed quite remotefrom the target area yet will permit higher power densities and smallerbeam width. This latter feature will permit greater freedom of movementin the work area and will also permit a single laser to power severalwork areas at the same time.

Therefore, an object of this invention is to provide a safe opticalfrequency waveguide.

Another object of the present invention is to provide an opticalfrequency power transmission system.

Another object of this invention is to provide a multichannel opticalfrequency waveguide structure.

Another object of this invention is to provide a low loss opticalfrequency waveguide.

Another object of this invention is to provide a relativelyindestructible transmission medium for optical energy.

Another object of the present invention is to provide an opticalfrequency power transmission system.

Another object of the present invention is to provide an opticalfrequency power distribution system.

Another object of this invention is to provide a waveguide capable ofhandling very high quantities of optical energy.

Another object of this invention is to provide an optical frequencywaveguide that effectively eliminates the conduction of unwanted lowfrequency signals.

Another object of the present invention is to provide a combinationpower and communication transmission system.

Another object of the present invention is to provide an op ticalfrequency bloodless surgical cutting and cell destroying system.

Another object of the present invention is to provide an opticalfrequency machining, drilling, and cutting tool.

Other objects and features of the present invention will be betterunderstood from the following specification when read in connection withthe attached drawings of which:

FIG. I shows an optical waveguide power transmiss on system.

FIG. 2 shows an optical frequency multichannel transmission system.

FIG. 3 shows a light beam of diameter D with its associated angles ofdispersion.

FIG. 4 shows diffraction of a ray as it strikes the boundary betweenmaterials of differing indices of refraction.

FIG. 5 shows a laser directing a beam of optical 'energy through a slabof homogeneous material and emerging therefrom.

FIG. 6 is a table of critical power and related factors computed forvarious materials.

FIG. 7 is a laser bloodless surgical cutting device.

An electromagnetic beam of optical frequency of sufficient power andproper diameter when directed into a dielectric medium will produce itsown dielectric waveguide and will propagate through said dielectricwithout spreading. This will occur in materials in which the dielectricconstants increase with field intensity but which are commonlyhomogeneous in the absence of said electromagnetic radiation.

Referring to FIG. 5, a laser 81 produces a high intensity beam 86, whichis directed through a slab of material 82, which can be solid, liquid,or gaseous, as will be described more fully as we proceed. The materialwithin the confines of the beam undergoes certain changes due to thefield produced by the laser beam. The material has a uniform dielectricconstant n,84, in absence of an electric field. However, within theconfines of the beam, the dielectric constant increases to some valuelarger than n,due to the electric field. This causes effectively a tubeof material with a higher dielectric constant to develop within theoriginal material corresponding (having a shape and diameter) to that ofthe electro tic bearn. Rays of energy 89 within the beam will hit theedges of this tube at the point of transition in the material having thehigher dielectric constant to that area having the lower dielectricconstant and is refracted such that all of the energy within the laserwas located very close to the target area to avoid diffracbeam isconstantly reflected from the sides of the tube formed by the beam.Therefore, this tube forms a waveguide which directs the energy withinthe beam and contains it therein. Summarily speaking, electromagneticenergy disturbs the dielectric constant of the material, causing thedielectric material to fonn a waveguide which traps the beam ofelectromagnetic energy.

The principles upon which the waveguide is formed and persists can bebetter understood from its mathematical development. Starting thisdevelopment we have Snell's Law, which is In FIG. 4 we see a ray oflight 75 passing through one medium having a dielectric constant of n,72 through a junction 78 n, sin 6a, sin 9,

' with another material having a dielectric constant n, 78 and the beamis refracted. Angle of incidence 74 and angle of refraction 6, 73results. Equation 1 summarizes this relationship. When the angle ofrefraction 73 becomes 90, as shown in FIG. 5, the ray is totallyreflected within the beam. When 9,=, then sin G I, and by substitutionwe have:

Equation 2 can be simplified by dividing both sides of the equation by nand we have:

Relating the latter equation to our system of self-generating waveguideas we have, n,=n the index of refraction of the entire homogeneousmaterial. Furthermore, we know the material within the waveguide willhave an index of refraction something greater than n,, which we willfind to be n,+n, E, therefore n =n,,+nE. The index of refraction isaffected by the square of the field illuminating it and a factor, nwhich we will find later is largely due to electrostriction. We willhave the following equation by substituting the above values in equation3:

sin nod-"4E Referring to FIG. 3, we have a beam of electromagneticenergy of diameter D52 shining upon a plane 53 having an opening 65 anda beam of diameter D55 emerges therefrom. We note that this beam has anangle of divergence a which is designated 59. This angle of divergence aappears in any electromagnetic wave in air, but FIG. 3 serves toillustrate this more effectively. If a ray 56 is parallel to a line 60which forms the angle of divergence 59 with the beam 54, it will form anangle of incidence 74 with the normal and an angle III critical which isits complement. For the purposes of our development, we can see that:

We can detennine from the examination of FIG. 3 that the beam of finitediameter D55 has an angle of defraction a. If we can maintain a ray 56within the beam such that it will have an angle ill critical, which isgreater than a, then the angle of incidence 6, 74 will be such that ray56 will be reflected back into and remain within beam 54. In FIG. 3, aand 6 being complementary angles then:

This is the result of our assumption that a is much less than 1.Substituting equations 8, 7, and 3, we have:

The E in equation 13 now represents the critical field which will resultin self-trapping of rays that make up the finite beam which we wish tocontain within its own waveguide.

Proceeding a step further, we must now have a power corresponding tothis critical field. Such a critical power would be equal to the productof the beam area times the Poyntiirg vec- ,0 tor of the field times thespeed of light. The area is represented by 11Dl4 and the Poynting vectorof the field is then the field El81rC in this equation represents theflow of power and the area presupposes that we have a uniformdistribution of power which is approximately correct and satisfactoryfor this expression. We therefore have:

The critical power by substituting E of equation 13 reduces to:

Substituting in equation 15 in which: A is the wavelength, n, is thedielectric constant of the material, and D is the diameter of the beam,we further Equation 17 is an approximation which conforms very closelyto more precise machine calculation. Indications are that a v 60beamabove a certain critical power, P will be trapped at a preselecteddiameter and not spread. This power level decreases with the square ofthe wavelength. For normal dielectric materials, the constant n, is suchthat the critical power for trapping is within one or two orders ofmagnitude at 10 for visible light, a power level commonly obtained inlaser beams. For radio waves the longer wavelength makes the criticos aZ cosrlr, (6) ca] power for such materials unattainable at present. Thenonlinear coeflicient n, is associated with high frequen- Knowmg alsothat CO5 by subsmuuon' cy Kerr effects involving molecular orientationwith electroscos a sin 9 (7) triction, and with nonlinear-ities due toelectronic polarizabili- By expansion we have:

cos a= 1 +some insignificant terms ty of the type which generates thirdharmonic waves in optical materials. For liquids, the first two effectsare of comparable size and the third much smaller, as is indicated inthe Table of FIG. 6. For solids, such molecular rotation is frozen outand electrostn'ctive efl'ectsdominate.

The beam when once trapped establishes a waveguide of appropriatecharacteristics for its own conduction, any weak wave of higherfrequency can also easily be shown to be conducted but not one of lowerfrequency. The dielectric properties of the waveguide are undisturbed tofirst order in the weak fields as long as the beat frequency between itand the initial wave is too high for the dielectric to respond. If thebeat frequency is lower, then one has a waveguide of modulateddielectric constant and solutions for the two simultaneous waves arevery complicated. This latter feature illustrates that a waveguide canbe established by a very strong continuous beam and that comparativelyweak signals of a higher frequency are conducted through that waveguide.Consequently, a

waveguide can be generated by one transmitter, while a second smallertransmitter is utilized for modulation and transmission of intelligenceover that waveguide.

Two waves whose frequency differences are too high for the dielectricresponse are more stably trapped than is a wave of a single frequency.This results from the increase in dielectric constants of the waveguideproduced by one wave which helps 2 form the waveguide and is relativelyunaffected by small perturbations of the second wave and vice versa.

The table of FIG. 6 gives values for n, for Kerr and forelectrostrictive effects, and a critical power calculated forelectrostrictive effects alone. For Kerr effects, n,=2/3 J where J isthe high frequency Kerr constant due to molecular rotation. F orelectrostriction,

where is the density, Bis the bulk modulus, and G is conductance.

Referring to FIG. 1, we see a laser beam trapped within a mediumcontained with a pipe 11. The laser l2 transmits energy to receivers I3and 14, thus illustrating a power distribution system. Some beam energyis tapped off by beam splitter 15. The beam 21 breaks up into components25 and 23. The first component of the beam travels to receiver 13, whilethe second travels to receiver 14. It must be noted again, however, thatthe energy of the beam 25, if it must continue any distance, must remainabove the critical power, P as outlined in the equations, to continue tohave its ability for selftrapping and thereby form its own waveguide.

Ordinarily power levels will be far above critical power levels and thefollowing techniques will be rendered unnecessary. However,communications systems where power levels may be slightly above criticalpower may require some correction. The gas pressure can be increased toreinstate selftrapping, which will be amplified further in subsequentparagraphs. Increasing the gas pressure on the other side of partition26 will permit beam to continue without spreading.

Referring to FIG. 2, we see a communications system utilizing a singlestructure or effectively a system of dielectric waveguides. Lasers 35,39, and 36 transmit through a pipe 31 to receivers 37, 43, and 38. Wenote that laser, 35 forms a beam 41 which produces its own waveguide andcontinues through to receiver 37. Laser 39 is inactive and produces nobeam and no resulting waveguide. Laser 36 produces beam 42 whicheffectively creates its own waveguide and continues to receiver 38. Theparticular advantage here is that hazardous conditions of transmittinghigh power laser signals are avoided by means of pipe 31. Furthermore,individual laser beams are not dispersed and do not intennodulate withone another, even though they travel a substantial distance. Obviously,a substantial number of independent channels can be sent over a'commonstructure.

The structure of both embodiments above is preferably a gas-filled steelpipe. The requisite power is very sensitive to pressure, as can be seenfrom an examination of the table of FIG. 6. Here we see that powerrequired for self-trapping in air at one atmosphere pressure is 100times greater than that mospheres. It is suggested that carbon dioxide(C0,) because it has a more favorable index of refraction be used undera pressure of I00 atmospheres. However, the pressure of the gas is afunction of beam power. When the beam power is high the pressure shouldbe low, and vice versa. In a distribution system, the main trunk lineshould have a gas pressure as low as one atmosphere and the branch linesshould have a gas pressure as high as I00 atmospheres. The lower the gaspressure, the lower the power absorption is. The absorption of power ofa gas, even at its higher pressure, is negligible when compared to thatof a solid or even a liquid.

Pulsed ruby lasers readily produce the power required for transmission;but this power is pulsed and average power transmission is consequentlyvery low. An array of gallium arsenide lasers together with a properfibre-optics system for gathering the power from each beam andultimately necking the beam down until a very intense continuous lightbeam is produced. The receiver for the power system can be any 0photovoltaic substance such as lead sulfide; but again an array ofgallium arsenide diodes which have a higher quantum efficiency and wouldtherefore be more desirable. Use of lasers is not essential. for thecoherence and polarization are not necessary to effective operation.White light if it had adequate total power could also be usedeffectively and can be obtained from a power flash tube or equivalentdevice.

Referring to FIG. 7, laser 91 directs optical frequency energy throughwindow 107 into pressure-tight container 92. Lens 105, is in the path ofsaid laser beam and is installed in movable lens holder l06 pivotedabout pedestal 99 attached to container 92. Lens holder 106 can beturned by handle 101, or can be rotated about pedestal 99, thus beingable to cause beam 96 to traverse the entire target 95. Bellows I02maintains container 92 pressure tight while lens holder 106 is raised upand down. Gas, such as carbon dioxide, fills chamber 92 at a pressure ofI00 atmospheres by way of inlet 93.

required for self-trapping in air at a pressure of I00 at- Lens 106concentrates the optical frequency beam emanating from laser 91 to afocal point midway between lens 106 and target 95. Intense beam 96 ofself-trapped optical frequency radiation is thereby formed. Target 95can be a pa-' tient's body ready to be incised, beam 96 acting as asurgical knife.

The above system can be readily adapted to machining. Micrometer controlof the lens system would have to be introduced. It should be noted herethat power density of the beam and critical power for self-trapping aredistinct from one another. Examining equation [7, critical power can bepreselected relative to wavelength, beam diameter and so 0 forth, suchthat a sufficiently high power density for the purpose of cutting isobtained. In other cases such as power transmission, power densityshould be kept low in order to minimize heating.

While we have described the above principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is only made by way of example and not as a limitationon the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:

I. An optical frequency communication system comprising:

source means for providing electromagnetic energy at a first frequencyand with critical power, definable as P,, and electromagnetic energy ata second frequency higher than said first frequency;

receiving means disposed from said source means; and

a structure disposed between said source means and said receiving meansand being filled with a material of a preselected dielectric constantwhich is a function of P and said first frequency whereby theelectromagnetic energies from said source means passing through saidstructure to said receiving means are trapped in a beam of a preselecteddiameter.

2. An optical frequency communication system according to claim 1wherein P, is not less than wherein A is the wavelength of said firstfrequency, c is the speed of light and n is in the order of X10 whereinX is not greater than 2.

3. An optical frequency communication system according to claim 1wherein said electromagnetic energy at said second frequency ismodulated and wherein said structure is a gastight structure and saidmaterial filling said structure is a gas at a preselected controllablepressure.

4. An optical frequency communication system according to claim 3wherein P, is not less than wherein A, is the wavelength of said firstfrequency, is the speed of light and n, is in the order of X where X isnot greater than 2.

5. An optical frequency communication system according to claim 4wherein said gas is carbon dioxide.

6. An optical frequency communication system according to claim 1wherein said source means comprises first and second lasers. said firstlaser providing said energy at said first frequency and said secondlaser providing modulated energy at said second frequency.

7. An optical frequency communication system according to claim 6wherein P, is not less than

1. An optical frequency communication system comprising: source meansfor providing electromagnetic energy at a first frequency and withcritical power, definable as Pc, and electromagnetic energy at a secondfrequency higher than said first frequency; receiving means disposedfrom said source means; and a structure disposed between said sourcemeans and said receiving means and being filled with a material of apreselected dielectric constant which is a function of Pc and said firstfrequency whereby the electromagnetic energies from said source meanspassing through said structure to said receiving means are trapped in abeam of a preselected diameter.
 2. An optical frequency communicationsystem according to claim 1 wherein Pc is not less than wherein lambdais the wavelength of said first frequency, c is the spEed of light andn2 is in the order of X106 wherein X is not greater than
 2. 3. Anoptical frequency communication system according to claim 1 wherein saidelectromagnetic energy at said second frequency is modulated and whereinsaid structure is a gastight structure and said material filling saidstructure is a gas at a preselected controllable pressure.
 4. An opticalfrequency communication system according to claim 3 wherein Pc is notless than wherein lambda is the wavelength of said first frequency, c isthe speed of light and n2 is in the order of X106 where X is not greaterthan
 2. 5. An optical frequency communication system according to claim4 wherein said gas is carbon dioxide.
 6. An optical frequencycommunication system according to claim 1 wherein said source meanscomprises first and second lasers, said first laser providing saidenergy at said first frequency and said second laser providing modulatedenergy at said second frequency.
 7. An optical frequency communicationsystem according to claim 6 wherein Pc is not less than where lambda isthe wavelength of said first frequency, c is the speed of light and n2is in the order of X106 where X is not greater than
 2. 8. An opticalfrequency communication system according to claim 7 wherein saidelectromagnetic energy at said second frequency is modulated and whereinsaid structure is a gastight structure and said material filling saidstructure is a gas at a preselected controllable pressure.
 9. An opticalfrequency communication system according to claim 8 wherein said gas iscarbon dioxide.